Subterranean wells have been drilled primarily to produce one or more of the following desired products for example fluids such as water, hydrocarbon liquids and hydrocarbon gas. There are other uses for wells but these are by far the most common. These desired fluids can exist in the geologic layers to depths in excess of 5,000 m below the surface and are found in geological traps called reservoirs where they may accumulate in sufficient quantities to make their recovery economically viable. Finding the location of the desirable reservoirs and drilling the wells present their own unique challenges. Once drilled, the wellbore of the well must be configured to transport safely and efficiently the desired fluid from the reservoir to surface.
Whether or not the desired fluid can reach surface without aid is a function of numerous variables, including: potential energy of the fluid in the reservoir, reservoir driver mechanisms, reservoir rock characteristics, near wellbore rock characteristics, physical properties of the desired fluid and associated fluids, depth of the reservoir, wellbore configuration, operating conditions of the surface facilities receiving fluids and the stage of the reservoirs depletion. Many wells in the early stages of their producing life are capable of producing fluids with little more than a conduit to connect the reservoir with the surface facilities, as energy from the reservoir and changing fluid characteristics can lift desired fluids to surface.
Typically fluids in a liquid phase cause the most problems when attempting to move the fluids vertically up the wellbore. Fluids in the liquid phase are much denser than fluids in a gaseous phase and therefore require greater energy to lift vertically. These fluids in the liquid phase can enter the wellbore in the liquid state as free liquids or they can enter the wellbore in the gas phase and later condense into liquid in the wellbore due to changing physical conditions. The liquids that enter the wellbore may be desirable fluids, such as hydrocarbon liquids or useable water, or they may be liquids associated with the desired fluids, for example, water produced with oil or gas. Often the liquids associated with the desired fluids must be produced in order to recover the desired fluid. Regardless of the desirability of the liquid, energy is required to transport the liquid vertically from the reservoir to surface. Optimizing the energy required through improved wellbore dynamics or with the aid of artificial lift has been an area of intense study and literature for those dealing with subsurface wells.
To improve the economics of a well, it is desirable to increase the production rate and maximize the recovery of the desired fluid from the well. Transportation of fluids from reservoir to surface, that is well bore dynamics, is one of the variables of the well that can be controlled and has a major impact on the economics of a well. One can improve the well bore dynamics by two methods—1) designing a wellbore configuration that optimizes and improves the flow characteristics of the fluid in the well bore conduit or 2) aiding in lifting the fluid to surface with artificial lift. Artificial lift can significantly improve production early in the life of many wells and is the only options for wells if they are to continue producing in the later stages of depletion. Regardless of whether the well can lift the desired fluids to surface on its own or requires artificial lift, the well bore dynamics should be reviewed continually as the variables change over the life of the well and the economics for the well need to be maximized.
The methods of improving flow characteristics include: proper tubing selection, plunger systems, addition of surface tension reducers, reduced surface pressures, downhole chokes and production intermitters. These methods do not add energy to the fluids in the well bore, and therefore are not considered artificial lift systems; however, they do optimize the use of the energy that the reservoir and fluids provide. These methods optimize the well bore dynamics and/or add energy to the fluid transportation process at the surface. Depending on the application, each of the different methods above has numerous models and configurations each having their own unique advantages and disadvantages.
There are numerous systems of artificial lift available and operating throughout the world. The more common systems are reciprocating rod string and plunger pumps, rotating rod strings and progressive cavity pumps, electric submersible multi-stage centrifugal pump, jet pumps, hydraulic pumps and gas lift systems. Again, depending on the intended application, each of the different systems has numerous models each having their own unique advantages and disadvantages. To fit in the category of artificial lift, additional energy not from the producing formation and fluids is input into the well bore to help lift fluids in the liquid phase to surface. The artificial lift systems listed above have been developed for water and hydrocarbon liquids as they require the greatest assistance when being transported to surface and provide the greatest economic incentive. They also have applications in lifting liquids that are associated with the gas in natural gas wells.
With the depletion of the world gas reserves there is a need to develop an artificial lift system that is better suited to removing liquids associated with natural gas production from the wellbore. These liquids, if not removed from the wellbore, will significantly limit the natural gas production rates as wells as the ultimate recovery of the natural gas reserves.
Other artificial lift systems have been designed and used based on injection of high-pressure gas. Gas lift is a common form of artificial lift and relies on injection of enough gas to reach the critical rate for removing liquids from the wellbore (Turner et al in 1969: Turner, R. G., Hubbard, M. G., and Dukler, A. E., 1969, “Analysis and Prediction of Minimum Flow Rate for the Continuous Removal of Liquids from Gas Wells,” J. Pet. Technol., 21(11), pp. 1475-1482.)
U.S. Pat. No. 5,211,242 by Malcolm W Coleman and J Byron Sandel outlines the complete removal of fluids from the well on each cycle, which requires large gas volume and therefore large associated equipment with pumping, for example large tubing, a large compressor, large power source valves, etc.
There is a need for pumps that can be installed and serviced without the use of a service rig using wireline or coiled tubing equipment and techniques, to allow for easy installation and servicing. There is a need for pumps that fit with existing technologies, services and equipment, and may fit with existing wellbore configurations with only minor modifications.